The present invention relates to a basic air-fuel ratio adjustment method and an apparatus for an internal combustion engine. More particularly, the invention relates to a basic air-fuel ratio adjustment method and an apparatus for adjusting the basic air-fuel ratio of the engine of the air-fuel ratio feedback control type to a desired value.
There is known a system of performing feedback control for maintaining an equivalent air-fuel ratio (if an air-fuel passage from the intake passage through the exhaust passage located upstream of an air-fuel ratio sensor is defined as a working fluid passage, the equivalent air-fuel ratio is defined as a ratio of the amount of air fed into the working fluid passage to the amount of fuel fed into the working fluid passage) within a predetermined range by compensating an error of the equivalent air-fuel ratio in accordance with an air-fuel ratio compensation signal fed back from the air-fuel ratio sensor disposed in the exhaust passage. In these systems, the basic air-fuel ratio is always maintained at a predetermined value by an air-fuel mixture supplying mechanism such as a carburetor or a fuel injection mechanism disposed in an intake system of the engine. The compensation of the error of the equivalent air-fuel ratio is generally carried out by increasing or decreasing the amount of fuel injected into the intake passage by means of the fuel injection valve, or by increasing or decreasing the amount of secondary air fed into the exhaust passage by means of a secondary air feeding mechanism.
Since it is desirable that the error of the equivalent air-fuel ratio to be compensated be as small as possible, a correct adjustment of the basic air-fuel ratio of an internal combustion engine is required.
One method for adjusting the basic air-fuel ratio is that of using a direct measurement method, such as the method of obtaining the basic air-fuel ratio by directly measuring both the amount of air introduced into the engine and the amount of fuel fed into the engine. However, since this direct measurement method requires a special measuring apparatus, it is difficult to adjust the basic air-fuel ratio at an ordinary engine service shop by using such direct measurement method.
Another method for adjusting the basic air-fuel ratio is that of using a simple carbon monoxide concentration detector (hereinafter referred to as a CO meter) which is always available at an ordinary engine service shop. This adjustment method is based on the fact that the concentration of the CO formation in the exhaust gas of the engine is related to the basic air-fuel ratio of the engine when the basic air-fuel ratio is on the rich side of the stoichiometric air-fuel ratio. Therefore, the adjustment of the basic air-fuel ratio by using the CO meter is always carried out during the rich fuel condition. In order to create this condition, the air-fuel ratio of the air-fuel mixture is intentionally shifted from its original position to the rich side of the stoichiometric air-fuel ratio, and then the air-fuel ratio is returned to its original position after the adjustment is completed.
However, since the concentration of the CO formation in the exhaust gas emitted from the engine which is provided with a secondary air supplying system for automatically controlling the amount of secondary air supplied to the engine in accordance with the concentration of the O.sub.2 formation in the exhaust gas and which is also provided with a catalytic converter having a good purifying efficiency, is extremely low even when the air-fuel ratio of the air-fuel mixture is on the rich side of the stoichiometric air-fuel ratio, the CO meter cannot be used for adjusting the basic air-fuel ratio.
Furthermore, in the latter method, since the air-fuel ratio of the air-fuel mixture is forcibly shifted to the rich fuel condition for adjusting the basic air-fuel ratio, the accuracy of the adjustment is not satisfactory. Therefore, by using this latter method, a large amount of harmful pollutants is sometimes emitted from the engine, and the operating characteristics of the engine also become worse.